1. Field of the Invention
This invention is directed to a new pavement preservation material that combines fine polymers, cement, fly-ash, aggregate, microfibers and water to form a flexible pavement surfacing overlay that exhibits superior performance, product uniformity, wear, shrinkage and surface friction properties.
2. Background
Most pavement preservation materials are bituminous; however, polymer modified cement materials are also available for use in pavement preservation and service life extension. With an increase in the cost of asphalt, the use of polymer modified cementitious materials have gained more acceptance, and such materials have been more widely used on a variety of road surfaces.
Current polymer modified cementitious materials are typically blended on site and make use of a dry blend of solids and cements, a liquid polymer emulsion and water. The product(s) are widely sold to contractors and combined according to approximate mixture parameters. There is a risk that the amount of the polymer and cement, the most costly mix components, can potentially vary according to the desired level of profitability on the job, rendering the service performance inadequate. Hence, quality, unless strictly controlled or supervised, can vary considerably.
Current polymers used for polymer modified cementitious overlays are mostly acrylic latex polymers which, when mixed with cement and aggregate blends, generate very high air voids. The high air voids will make the product porous. The porous cured product will neither fully protect asphalt surfaces from hydrocarbons, nor prevent the ingress of surface water into the underlying supporting layers. The high air void content (and associated lower density of the placed product) will also result in a product that is more susceptible to wear, abrasion and friction loss.
Further, the polymer modified cementitious materials are installed on a pavement surface, the polymer tends to form a thin polymer film on the surface despite thorough mixing prior to placement. Depending on the ambient temperatures and humidity, the accelerated curing of this thin polymer film can lead to the development of tension cracks as this layer shrinks more rapidly than the underlying mix. The most commonly applied method to reduce the incidence of these crack formations is to simply break the surface tension by spreading a thin layer of sand on the surface. However, this is not well received; firstly, it is a second process that needs to be performed after installing the polymer modified layer; and, secondly, it is very difficult to control the broadcasting of the sand in a single closed traffic lane being overlaid.
Also, to increase the skid resistance of the polymer modified cements, the use of a topically applied sand is usually required. Although increasing the sand content in the mix can also aid in increasing the skid resistance, the current range of polymer modified cement pavement overlay mixtures typically have gap graded sands, i.e., a blend of 2 to 3 specified sizes, which means that the incidence of segregation (the heavier, coarser, sand settling well below the wearing friction surface) prevents the sand from having a significant effect on the surface friction.
The current range of polymer modified cementitious surfacing materials have demonstrated that they are susceptible to even more segregation when necessarily applied in thicker lifts, such as in the wheelpaths that have rutted ½ inch or more, and over cracks that are greater than ¼ inch wide and deep. When the mix has segregated, the larger particles settle to the bottom of the layer, and the fines remain at the top. In addition, when sand segregation does occur, the discontinuous sand profile provides poor flexural performance and cracks form when the layer is subjected to trafficking.
Current polymer modified cementitious surfacing materials are applied during the day to take advantage of sunlight or daylight to help achieve hydration of the cement and therefore allow the road to be opened to traffic within a reasonable timespan. The night time cure times can be as long as 4 to 5 hours depending on the temperature and humidity. Heaters and air blowers have been tried to aid in accelerating the curing process, but such usage is not recommended as the removal of water will reduce the water available for hydration and will weaken the strength aspect contributed by the cement in the mix. This slow cure is difficult to address inasmuch as most commercially available cement accelerators do not work with polymer modified cements.